1. Field of Invention
The present invention relates to a driving method for a liquid crystal display, and more particularly, to a pre-charge method for a liquid crystal display, wherein a pre-charge voltage value is applied to a scan line before a scan signal is electrically coupled to the neighboring pixel via a storage capacitor.
2. Description of the Related Art
A liquid crystal display advances, not only in dimension thereof, but also in larger variety of image types. For example, most LCDs are used for still images on a personal computer or a word-processing product, yet currently most products are capable of displaying motion pictures, such as LCD television. Since a LCD is rather smaller and thinner than conventional cathode ray tube television, and is not space consuming after installed, it is foreseeable that LCD is getting more and more popular for human life.
Referring to FIG. 1, a conventional LCD structure is illustrated. The LCD includes a first layer glass substrate and a second layer glass substrate, wherein the CLD panel 100 is for displaying an image. A plurality of scan lines 101 (n lines as shown in the figure) and signal lines (m lines as shown in the figure) are disposed over the first layer glass substrate in grid-like arrangement. The thin film transistors 103 serving as switches are disposed in vicinity of cross points of each scan line 101 and signal line 102.
A gate of each TFT 103 is coupled to one of the scan lines 101, a source of which is coupled to one of the signal lines 10, and a drain of which is coupled to one pixel electrode 104. Said second layer of glass substrate is disposed against the first layer of glass substrate, formed with a common electrode 105 comprised of such as ITO (indium tin oxide). The liquid crystal is stuffed between the first layer of glass substrate and a second layer of glass substrate.
Scan lines 101 and signal lines 102 are respectively coupled to a scan line driving circuit 106 and a signal line driving circuit 107. The scan line driving circuit 106 drives a large voltage level to the n scan lines 101 and switches on each of the TFTs 103 associated with the scan lines 101. Since the scan line driving circuit 106 is at a scanning state, the signal driving circuit 107 outputs representative image having gradation voltage for m signal lines, so that the voltage is coupled to the TFT 103 via the scan line 102 to write the corresponding pixel electrode 104. The written pixel electrode 104 has a gradation voltage differed with a voltage level of the common electrode 105 for controlling brightness of transmitted light.
Referring to FIG. 2, a waveform diagram of the conventional LCD from the scan line driving circuit 106 to the scan line 101, and from the signal line driving circuit 107 to the signal line 102 is illustrated. Where VG1 to VGn are scanning signals of each of the scan lines 101. It is clearly seen that each of the intervals of VG1 to VGn provides only one scan line 101, and sequentially to all of the scan lines 101. Where VD is a data signal of a gradation voltage outputted to the signal line 102. The strength of the data signal (amplitude of the voltage level thereof) is determined by the image to be displayed. Vcom is a voltage level of the common electrode 105, usually keeping invariant with time.
If serving the aforementioned conventional LCD as motion image display, e.g. current television system, a large amount of motion images is required. However, according to hold-type addressing method of the LCD the displaying light is retained for a field period long, from data written to the pixel to writing operation for the next period. Therefore edge blur is incurred. In order to solve the problem, a lot of improvements were proposed, such as “A Black Stripe Driving Scheme for Displaying Motion Pictures on LCDs” by T. Nose, M. Suzuki, D. Sasaki, M. Imai, and H. Hayama disclosed by NEC in Society for Information Display in 2001. The structure of which circuitry is complicated, and requires special gate input waveform and higher data frequency. On the other hand, RC delay effect is induced from gate circuitry, so that not applicable to panels with large dimension and higher resolution.
Furthermore, “A Novel wide-Viewing-Angle Motion-Picture LCD” by G. Nakamura, K. Miwa, M. Noguchi, Y. Watbale, and J. Mamiya is disclosed by IBM Japan in SID in 1998. The structure thereof is divided into upper half portion and lower half portion, so that two data driving IC are required. Not only higher cost is required, transmittance of liquid crystal cell is drastically lowered since black-insert-ratio is merely fixed at 50%.
According to the conventional schemes mentioned above and technology that is known to the skill in the art, a lot of problems do exist, i.e. panels are not suitable for large dimension or high resolution, or only capable of row inversion driving method.
In order to implement a panel with large dimension and high resolution, manufactures in the relevant industry proposes another LCD structure 300, where an equivalent circuit diagram is illustrated in FIG. 3. For simpler description, only a portion of the structure is illustrated therein. The LCD structure 300 includes a scan line 301(n) and 301(n+1), and a signal line 302(n) and signal line 302(n+1). TFT 303(n) and 303(n+1) thus correspond to the signal line 301(n) and signal line 302(n).
The TFT, e.g. 303(n), of the LCD structure 300 having wide viewing angle, is coupled to the scan line 301(n) via a agate, and a source thereof is coupled to the signal line 302(n). A drain of the TFT 303(n) is coupled to the gate thereof via a gate/drain capacitor Cgd, coupled to the scan line 301(n+1) via a storage capacitor Cst, and coupled to the common electrode is a liquid crystal capacitor Clc.
Similarly for the neighboring TFT 303(n+1), a gate is coupled to the scan line 301(n+1), a source is coupled to the signal line 302(n+1). A drain of which is coupled to the gate thereof via a Cgd, coupled to the previous scan line 301(n) via a Cst, and coupled to the Vcom via a Clc.
The driving method of the LCD structure 300 follows the waveform diagram illustrated in FIG. 4, which is a capacitively couple driving method. According to the figure, voltage levels Vg(n) and Vg(n+1) respectively supply the scan lines 301(n) and 301(n+1), and voltage levels Vs(n) and Vs(n+1) respectively supply the signal lines 302(n) and 302(n+1). The driving method includes four gate voltage values, i.e. TFT on voltage, TFT off voltage, Vg(+) and Vg(−). First, the signal voltage Vs is coupled to the pixel electrode via the TFT. After charging the pixel, the capacitively coupled driving voltage including a previous or a next stage of scan line is transmitted to the pixel electrode Vg(+) and Vg(−) fed back from the Cst.
The driving method is advantageous that pixel voltage can be larger than that supplying to the signal, i.e. the signal value can be tiny. In such a LCD driving structure, since neighboring scan lines are constantly provided voltage value with opposite polarity (i.e. column inversion driving structure), therefore, voltage level fluctuation due to capacitance between signal lines and the common electrode through this driving method. This driving structure can also eliminate vertical cross-talk caused by parasitic capacitance between signal lines and the pixel electrode.
Another conventional LCD structure is disclosed in “Response Time Improvement of OCB mode TFT-LCDs by using Capacitively Coupled Driving Method” by Kenji Nakao, Shoichi Ishihara, Yoshinori Tanaka, Daiichi Suzuki, Tsuyoshi Uemura, Keisuke Tsuda, Noriyuki Kizu and Junichi Kobayashi by Matsushita Electric. Co. in SID 2000, wherein an optically self-compensated birefringence, OCB, with rapid response is proposed for the LCD. Capacitively Coupled voltage is used in this driving method, where a voltage level is coupled to the neighboring pixel electrode via storage capacitor between neighboring scan line and pixel electrode, so as to overdrive the pixel to obtain rapid response.
In addition, another conventional LCD structure is applicable to lower power consumption. For example “Low Power Driving Options for an AMLCD Mobile Display Chipset” by Jason Hector and Pascal Buchschacher is disclosed in SID 2002, where lower power consumption of LCD is achieved with the proposed structure thereof. In order to narrow the operating voltage range, the driving method uses capacitively coupling method to pre-charge a pixel electrode via Cst between neighboring scan line and pixel electrode. For example, during positive field, a positive voltage of (Vsat+Vth)/2 is applied, where Vsat is saturation voltage of the pixel electrode, and Vth is threshold voltage thereof. Hence voltage range is narrowed so as to lower power consumption.
The foregoing LCD and driving method are advantageous, yet merely applicable to column inversion driving method or row inversion driving method. However, larger and larger dimension of LCD is required, where the driving method is thus developed as dot inversion driving method as opposed to conventional driving method that are outdated.